The stator vanes and the rotor blades in a gas turbine are exposed to the high temperature of the working fluid passing the vanes and the blades. Due to the high temperature a significant oxidation of the base alloy can occur at the leading edge of an aerofoil of the stator vane or, the rotor blade. Furthermore, oxidation at the inner platform of the inner shroud and/or outer shroud of the stator vane or the rotor blade can also occur. Such degradation is currently the life limiting mechanism on this component.
To address this problem, it is known to provide the components subject to high temperature with a Platinum-Aluminide (PtAl) or MCrAlY first coating applied to the base material substrate of the component. Thermal Barrier Coating (TBC) ‘patches’ provided over the first coating afford additional high temperature protection to local areas of the stator vane or rotor blade that would otherwise be life-limiting for the component. TBC patches are typically made of ceramic material. At the patches location it is in particular known to apply a High Velocity Oxy-Fuel (HVOF) MCrAlY coating to the base material substrate, instead of the PtAl coating, for promoting the bonding of the TBC coating. The TBC coating is then applied on top of the HVOF MCrAlY coating. The use of an HVOF MCrAlY layer in direct contact with the TBC patches is deemed necessary to prevent premature delamination of the TBC coating.
Such procedure, involving two different coatings over the base material substrate, i.e. a HVOF MCrAlY coating below the TBC patches and a PtAl coating over the rest of the substrate, is considered not practical and not cost effective.